It has been known to protect zinc surfaces such as galvanized steel by using silicate treatments, e.g., a coating of potassium water glass, to provide corrosion resistance for the zinc surface. Such coatings ostensibly compare favorably with zinc substrates that are chromate treated.
It is also known in the protection of zinc surfaces, which have been first treated by a traditional chromate coating, to topcoat the treated surface with colloidal silicas or silicate solutions. this further protection against white rusting can be obtained with films such as from silicate solutions of sodium silicate and/or potassium silicate. In addition to retarding white rusting, the topcoating can also retard staining, as has been discussed in Japanese Patent Disclosure No.: Showa 53-125239.
Moreover, the application of protective coatings of silicate directly on iron surfaces has been previously shown. Such may be achieved by direct application of silicate materials to the iron surface or by precipitation of collodial silicas onto an iron surface. The transitory corrosion protection thereby provided the ferrous substrate is well known.
Further in the protection of ferrous surfaces, it has been known to mix hexavalent chromium compounds and silicate materials in the same coating composition. These can typically be emulsions containing resinous materials. Emulsives may include polyacrylic acid, and coating operations can proceed in conventional manner to achieve corrosion protection for the ferrous surface.
A variety of at least substantially resin free, chromium-containing coatings for protecting ferrous substrates are also known. Of especial interest are those which contain particulate metal. Representative coating compositions can be relatively simplistic such as the compositions that may essentially contain chromic acid, and particulate metal in an alcohol medium, as disclosed in U.S. Pat. No. 3,687,738. Other, more complex compositions such as shown in U.S. Pat. No. 3,907,608 may contain the pulverulent metal and hexavalent-chromium-providing substance in a liquid medium comprising water plus high-boiling organic liquid. Such coatings over ferrous surfaces provide a highly desirable protection against red rust upon exposure of the surface to salt solution.